Wait for all uploads to complete

[pregress]

This change introduces a fix for #48 by using a CountDownLatch to wait until all files are uploaded.

Fixes #48

Testing done

Ran the existing integration tests.
Tested with: https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/azure-artifact-manager/181.v233fd271dd66/azure-artifact-manager-181.v233fd271dd66.hpi

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[pregress]

@strangelookingnerd or @gavinfish could you have a look? And if possible merge and release this?

Disclaimer: I'm not a Java dev, and noticed there is no coverage, also not for the existing code: #17 (comment)

[timja]

Thanks!